A nacelle for a turbofan propulsion system on a typical commercial airliner is structure which surrounds the engine providing smooth aerodynamic surfaces for airflow around and into the engine. The nacelle also helps define a bypass air duct through the propulsion system. The nacelle may also include a thrust reverser which can deploy and provide reverse thrust to help slow the aircraft after touchdown. Part of the nacelle is an aerodynamic cowl that surrounds the engine core aft of the fan, and forms the radial interior surface of the bypass air duct. It forms a generally enclosed space around the core of the engine called the core compartment. Thrust reverser halves may hinge open in order to provide access to the core compartment.
Nacelles may use bulb seals in many locations, such as between the thrust reverser halves and the portions of the pylon or nacelle contacted by the thrust reverser halves. When elastomeric bulb seals are new, the seals may be overly stiff, which may decrease the ability of the seal to compress and form a complete seal, and which may cause deformation or deflection of adjacent components. Over time, the seal may soften beyond the desired stiffness, which may also prevent the seal from forming a complete seal.